Generally, liquid developers are used in a form that colored resin particles containing a coloring agent such as a pigment are dispersed in an electrically insulating medium. As the process for production of such liquid developers, there are various methods such as (1) polymerization method where monomer components are polymerized in an electrically insulating medium with a coloring agent being dispersed therein to form colored resin particles; (2) wet pulverization method where after kneading a coloring agent and resin at a temperature not less than the melting point of the resin, dry pulverization is conducted, and the pulverized powder is subjected to wet pulverization under the presence of a dispersing agent in an electrically insulating medium; and (3) precipitation method (coacervation method) where from a mixture of a coloring agent, a resin, a solvent which can dissolve the resin and an electrically insulating medium which cannot dissolve the resin, said solvent is removed to cause the resin to precipitate, whereby a colored resin particle is dispersed in the electrically insulating medium.
However, the polymerization method (1) has a problem that it requires a step for eliminating residual monomers after polymerization. Further, the wet pulverization method (2) has a problem that since coloring agents are not completely contained in a resin, coloring agents aggregate together to result in a nonuniform particle size of colored resin particle, the resultant liquid developer is insufficient in dispersion stability and optical properties. Moreover, the precipitation method (3) has a problem that coloring agents aggregate together in precipitation of resin, making a particle bulky, and the same problem as in the wet pulverization method (2) that the resultant liquid developer is insufficient in dispersion stability and optical properties.
In view of the situations, in order to solve the above-described problems in the precipitation method (3), there has been proposed a method that after dissolving a resin in a solvent capable of solving the resin, mixed with an electrically insulating medium in the coexistence of a coloring agent and a dispersing agent, further, the solvent is removed from the mixture, thereby to disperse colored resin particles in the electrically insulating medium (see Japanese Unexamined Patent Publication No. 2003-241439).
In the progress of various printing technologies, however, to compete with other methods and obtain advantages, in recent liquid developers, high concentration of the liquid developer itself and high-resolution image of printing have increasingly become the most desired performances. Hence, to satisfy these required performances, colored resin particles must be minute and concentrated, but now it is the extremely difficult techniques to produce a minute colored resin particle and also to disperse it stably at high concentration, there have been desired a process for production of a new liquid developer to realize them.